This application is based upon Japanese Patent Application No. 2000-343664 filed on Nov. 10, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to manufacturing method for producing single-crystal silicon carbide that has low defects and high quality, and apparatus suitable for the same.
2. Related Art
Silicon carbide (SiC) has been developed as a semiconductor substrate for a power device because SiC has characteristics such as withstanding high voltage and high electron mobility. Generally, the single-crystal SiC is produced by a single crystal growth method called sublimation (the Modified Lely Method).
In the Modified Lely Method, silicon carbide source material is held in a graphite crucible, and a seed crystal is held in the graphite crucible to face the source material. The source material is heated to approximately 2200 to 2400xc2x0 C., generating sublimed gas. The temperature of the seed crystal is kept lower than the source material by several tens up to several hundred degrees Celsius, causing the sublimed gas to recrystallize on a growth surface of the seed crystal and forming single SiC crystals.
However, there is a limit to growth with the Modified Lely Method as the source material decreases with the growth of single SiC crystals. Although new source material can be added, SiC is sublimed at a rate in which the ratio of Si to C is more than one, so that the concentration of the sublimed gas vacillates when the new source material is added in the growth process, thereby preventing high quality single SIC crystal growth.
An epitaxial growth method of single SiC crystals by Chemical Vapor Deposition (CVD) is disclosed in JP-A-11-508531 (U.S. Pat. No. 5,704,985). FIG. 4 shows a schematic cross sectional view of an apparatus for the epitaxial growth method described in the above-mentioned publication. As shown in FIG. 4, a susceptor 2 is used as a crucible and is disposed approximately at the center of a cylindrical case 1. The susceptor 2 is composed of high-purity graphite or the like. The SiC single crystal substrate is a seed crystal and is disposed on an upper, inner surface of the susceptor 2 to promote epitaxial growth. A heater 4 is provided at an outside portion of the case 1 to heat gases inside the susceptor 2.
The space surrounding the susceptor 2 is filled with a thermal insulator 5 composed of porous graphite. An inlet conduit 6, which is formed by the thermal insulator 5 and which has a funnel shape, is located under a bottom of the susceptor 2. A supplying portion 7 is located at a bottom of the case 1 to supply a mixture gas while outlet conduits 8 are disposed at a top of the susceptor 2 to exhaust the mixture gas. A conduit 9 is disposed at the upper side of the case 1 and communicates with the outside of the case 1.
In the apparatus described above, the mixture gas supplied by the supplying portion 7 is transferred to the susceptor 2 through the inlet conduit 6 formed by the thermal insulator 5. The gas mixture is heated by the heater 4 and epitaxial growth occurs on the seed crystal 3 to form a single silicon carbide crystal. Any remaining gas mixture is exhausted through the outlet conduits 8 disposed at the top of the susceptor 2 and the conduit 9, which is disposed at the upper side of the case 1.
However, in the CVD manufacturing method for producing a single silicon carbide crystal the inlet conduit 6 is composed of the graphite 5, and the heat from the susceptor 2 tends not to be transmitted to the inlet conduit 6. Therefore, the temperature of the gas mixture in the inlet conduit 6 is Low and the gas mixture is heated up rapidly when transferred to the susceptor 2. When the gas mixture is rapidly heated as described above, high quality single, silicon carbide crystals cannot be obtained.
Therefore, the temperature of the gas mixture should be increased in the inlet conduit 6 to a predetermined temperature and then supplied to the susceptor 2. However, when the temperature of the gas mixture is 500xc2x0 C. or more, Si deposits may form on the wall surface of the inlet conduit 6. When the gas mixture is heated to a temperature where Si and C react with each other, SiC deposits may form on the wall surface of the inlet conduit 6. The deposits may obstruct the inlet conduit 6.
Furthermore, there is the possibility that, in a path 8, at an upper surface of the susceptor, which is narrow or in a path 9, which communicates with the outside of the upper chamber, Si which remains in the mixture gas or SiC as a product by reaction may deposit and block those paths.
The present invention addresses the above-mentioned problems, and an object thereof is to provide a manufacturing method of single silicon carbide crystals that is capable of preventing blockage caused by the mixture gas, and as another of its objects to provide an apparatus for the same.
An object of the present invention is to provide a manufacturing method for producing silicon carbide crystal using source gases capable of preventing blockage due to a gas mixture. A second object of the present invention is to provide an apparatus for providing the same.
According to a first aspect of the present invention, the method is characterized in that a temperature gradient occurs in an inlet conduit for introducing a gas mixture so that the temperature of the inlet conduit rises toward a portion of the inlet conduit that is closest to the crucible.
With this method, the mixture gas is not heated rapidly after it is introduced into the crucible. The mixture gas, which is heated up when passing through the inlet conduit, is introduced into the conduit. Therefore, silicon carbide single crystals having good quality are formed.
Even if the mixture gas is heated up to a temperature at which Si or SiC or the like can deposit in the crucible, when the mixture gas moves to a portion of the inlet conduit that is at a temperature that is greater than a temperature at which Si and SiC can sublime, the deposit is prevented.
In the present invention, the temperature gradient is such that the mixture gas moves to a portion having higher temperature, and the volume of the mixture gas expands, so that the velocity of the mixture gas in the inlet conduit increases as the temperature rises.
Therefore, the mixture gas can move to a higher temperature portion quickly, and a manufacturing method for producing silicon carbide single crystals is provided that is capable of preventing the inlet conduit from being plugged by the mixture gas.
Preferably, the inlet conduit has a hole in which the sectional area thereof decreases at locations closer to the crucible. According to this feature, since the path of the mixture gas tapers and becomes smaller toward the crucible, the velocity of the gas mixture increases as the gas mixture approaches the crucible. Therefore, the velocity of the gas mixture gas is increased more than that in a case where the sectional area of the hole in the inlet conduit is constant.
Preferably, the mixture gas is mixed with a carrier gas to be introduced into the crucible through the inlet conduit.
According to this feature, the velocity of the mixture gas is increased in the inlet conduit. Therefore, the inlet conduit is not blocked by SiC or Si deposits from the gas mixture.
Preferably, the temperature at an exhaust portion of the crucible, through which the mixture gas is exhausted from the crucible is set higher than that at an introducing portion of the crucible, through which the mixture gas is introduced into the crucible.
Since deposits caused by the mixture gas tend to form on a portion of the conduit that has a lower temperature, the exhaust portion is prevented from being plugged with the deposits by increasing the temperature at the exhaust portion.
Preferably, a movable temperature raising means and an x-ray apparatus are provided outside the crucible to reduce deposits caused by the mixture gas.
Preferably, the temperature of a room for passing the mixture gas therein that is exhausted from a growth room, where silicon carbide single crystals grow on the silicon carbide single-crystal substrate, is set lower than that of the growth room, so that deposits due to the mixture gas are formed in the room.
Deposits caused by the mixture gas tend to be formed at low temperature locations. Therefore, by forming the deposits in this room, an exhaust conduit for exhausting the mixture gas to the outside of the apparatus is prevented from being plugged with the deposits. Incidentally, since the deposits are formed on a wall of the room, a path of the gas remains unobstructed.
The apparatus of this invention has a crucible in which a silicon carbide single-crystal substrate is disposed as a seed crystal, and a mixture gas containing Si and C is introduced, so that silicon carbide single crystals grow on the silicon carbide single-crystal substrate. Specifically, an inlet conduit has a temperature gradient so that the temperature of the inlet conduit at points near the crucible is higher than that at points further from the crucible.
A first heat insulator or second heat insulator or the like may be provided at a location halfway along the inlet conduit or between the inlet conduit and the crucible to adjust the transmission of heat from the crucible to the inlet conduit.
A plurality of materials, such as graphite, quartz, or metal or the like, each of which has different thermal conductivity, can be employed in the inlet conduit to make the temperature gradient therein suitable. In this case, the graphite should be located nearest to the crucible because of its high heat resistance. Preferably, the quartz is located between the graphite and the metal.
Preferably, the sectional area of a hole formed in the inlet conduit decreases at locations closer to the crucible. Thus, the velocity of the mixture gas is increased.
Preferably, TaC is formed on an inner surface at least at an outlet portion in the inlet conduit.
Preferably, the surface roughness is regulated to be, for example, 7 xcexcm or less.
Preferably, an inner surface of the inlet conduit may be a polished surface, or a mirror surface.
Preferably, a protrusion is formed inside the crucible, and the protrusion has a communicating path, which connects the inlet conduit to the growth room. An opening area of the communicating path is tapered and increases in the direction approaching the silicon carbide single-crystal substrate, and a wall surface of the communicating path has a convexity and concavity. With this feature, fallen deposits due to the mixture gas are trapped in the concavity, and the inlet conduit is prevented from being plugged with the dropped deposits.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.